Along with the recently increased demand for portable electronic products such as laptop computers, video cameras, mobile phones, and so on, the development of electric vehicles, energy storage batteries, robots, satellites, and so on started in earnest and led into active researches on high-performance secondary batteries capable of repeated charging and discharging.
Currently, commercially available secondary batteries comprise nickel cadmium, nickel hydrogen, nickel zinc and lithium secondary batteries. Among them, lithium secondary batteries have drawn much attention because of little memory effect to allow unrestrained charging and discharging, as well as very low self-discharging rate and high energy density, compared to nickel-based secondary batteries.
Meanwhile, these secondary batteries are used as single secondary batteries, but in order to provide high-voltage and/or high-capacity power storage, the secondary batteries are also frequently used in a form in which a plurality of secondary batteries are connected in series and/or in parallel, i.e., in a form of a battery pack including a battery management apparatus for controlling the overall charging and discharging operations of the secondary batteries therein.
For such power storage apparatus that uses high-voltage, high-capacity secondary battery, it is very important to remain insulated. If the insulation of the battery pack is not maintained, leakage current will be generated, causing numerous problems. Specifically, the leakage current can shorten service life of the battery pack, cause malfunction of the electrical equipment in which the battery pack is used, and cause safety accident such as electric shock.
In order to prevent the generation of the leakage current, a battery pack is provided with an insulation resistance measuring device that can monitor the insulation resistance. Such insulation resistance measuring device frequently or periodically measures the insulation resistance of the battery pack, thus allowing a battery pack management apparatus to monitor the insulation state.
FIG. 1 schematically illustrates a configuration of a battery pack provided with a related insulation resistance measuring device.
Referring to FIG. 1, the battery pack includes a battery assembly 20 which is an assembly of one or more battery cells 21. Additionally, the insulation resistances RLeak(+), RLeak(−) are provided on a positive electrode terminal of the battery assembly 20 and on a negative electrode terminal of the battery assembly 20, respectively. The insulation resistances RLeak(+), RLeak(−) can be equivalent resistances that represent the insulation state of the battery pack. When the insulation state of the battery pack is well maintained, the resistance value of the insulation resistance will have a sufficiently high value, whereas the resistance value of the insulation resistance will have a low value below a threshold when the insulation state of the battery pack is broken.
Referring to FIG. 1 again, an insulation resistance measuring device 10 is connected to the positive electrode terminal and the negative electrode terminal of the battery assembly 20. The insulation resistance measuring device 10 includes a test resistor 11 therein, and a voltage measurement unit 12 for measuring a voltage applied to the test resistor 11. The insulation resistance measuring device 10 calculates insulation resistance RLeak(+) of the positive electrode side and insulation resistance RLeak(−) of the negative electrode side by using voltage value measured through the voltage measurement unit 12.
The problem is that the battery pack may have a presence of parasitic capacitor. In an equivalent circuit of a parasitic capacitor component modeled in the same manner as the insulation resistance, the parasitic capacitor component can be expressed as a capacitor connected in parallel to the insulation resistance.
FIG. 2 schematically illustrates an equivalent circuit of a battery pack representing an insulation resistance and a parasitic capacitor component. That is, the battery pack illustrated in FIG. 2 includes insulation resistances RLeak(+), RLeak(−), and parasitic capacitors CP(+), CP(−) connected in parallel to the insulation resistances. As such, when parasitic capacitors CP(+), CP(−) are present, it is difficult for the insulation resistance measuring device 10 to measure voltage properly. That is, due to delay occurred after switching by the parasitic capacitors CP(+), CP(−), a predetermined time has to elapse until the voltage measurement unit 12 measures voltage correctly.
In other words, in an ideal situation where no parasitic capacitors CP(+), CP(−) are present, the voltage value has a constant value immediately after switching. However, in a situation where there are parasitic capacitors CP(+), CP(−), the voltage reaches a stable state after a certain period of time.
Accordingly, in order to measure correct voltage value according to the related technology, it is necessary that a sufficient time passes to allow the voltage after switching to reach a stable state. In other words, the insulation resistance measuring device 10 needs an additional delay time to calculate insulation resistances RLeak(+), RLeak(−).
To summarize, according to related art, the presence of the parasitic capacitor components CP(+), CP(−) in the battery pack inhibits immediate calculation of the insulation resistances RLeak(+), RLeak(−).